Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency (18 percent) crystalline solar cells for space applications. When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially defect-free germanium and particularly silicon crystals, and then turn them into extrinsic materials with p-type and n-type conductivity regions therein. This was accomplished by diffusing into such crystalline material parts per million of donor (n) or acceptor (p) dopant materials introduced as substitutional impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type. The fabrication processes for making p-n junction crystals involve extremely complex, time consuming and expensive procedures. Thus, these crystalline materials useful in solar cells and current control devices are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, and when p-n junctions are required, by doping such single crystals with extremely small and critical amounts of dopants.
These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an impossible economic barrier to the large scale use of the crystalline semiconductor solar cells for energy conversion. Further, crystalline silicon has an indirect optical edge which results in poor light absorption in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the single crystal material is replaced by poly-crystalline silicon with cheaper production processes, the indirect optical edge is still maintained; hence the material thickness is not reduced. The polycrystalline material also involved the addition of grain boundaries and other problem defects.
In summary, crystal silicon devices have fixed parameters which are not variable as desired, require large amounts of material, are only producible in relatively small areas and are expensive and time consuming to produce. Devices based upon amorphous silicon can eliminate these crystal silicon disadvantages. Amorphous silicon has an optical absorption edge having properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystalline silicon. Further, amorphous silicon can be made faster, easier and in larger areas than can crystalline silicon.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be readily doped to form p-type and n-type materials where p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years, such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films are normally four-fold coordinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making p-n junctions for solar cell and current control device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon and germanium, W. E. Spear and P. G. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on "Substitutional Doping of Amorphous Silicon", as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196, 1975, toward the end of reducing the localized states in the energy gap in amorphous silicon or germanium to make the same approximate more closely intrinsic crystalline silicon or germanium and or substitutionally doping the amorphous materials with suitable classic dopants, as in doping crystalline materials, to make them extrinsic and of p or n conduction types.
The reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein a gas of silane (SiH.sub.4) was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and deposited on a substrate at a substrate temperature of about 500.degree.-600.degree. K. (227.degree.-327.degree. C.). The material so deposited on the substrate was an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material a gas of phosphine (PH.sub.3) for n-type conduction or a gas of diborane (B.sub.2 H.sub.6 for p-type conduction were premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used was between about 5.times.10.sup.-6 and 10.sup.-2 parts per volume. The material so deposited included supposedly substitutional phosphorus or boron dopant and was shown to be extrinsic and of n or p conduction type.
While it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of making the electronic properties of the amorphous material approximate more nearly those of the corresponding crystalline material.
The incorporation of hydrogen in the above method not only has limitations based upon the fixed ratio of hydrogen to silicon in silane, but, more importantly, various Si:H bonding configurations introduce new antibonding states which can have deleterious consequences in these materials. Therefore, there are basic limitations in reducing the density of localized states in these materials which are particularly harmful in terms of effective p as well as n doping. The resulting density of states of the silane deposited materials leads to a narrow depletion width, which in turn limits the efficiencies of solar cells and other devices whose operation depends on the drift of free carriers. The method of making these materials by the use of only silicon and hydrogen also results in a high density of surface states which affects all the above parameters.
After the development of the glow discharge deposition of silicon from silane gas was carried out, work was done on the sputter depositing of amorphous silicon films in the atmosphere of a mixture of argon (required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon film. This research indicated that the hydrogen acted as an altering agent which bonded in such a way as to reduce the localized states in the energy gap. However, the degree to which the localized states in the energy gap were reduced in the sputter deposition process was much less than that achieved by the silane deposition process described above. The above described p and n dopant gases also were introduced in the sputtering process to produce p and n doped materials. These materials had a lower doping efficiency than the materials produced in the glow discharge process. Neither process produced efficient p-doped materials with sufficiently higher acceptor concentrations for producing commercial p-n or p-i-n junction devices. The n-doping efficiency was below desirable acceptable commercial levels and the p-doping was particularly undesirable since it reduced the width of the band gap and increased the number of localized states in the band gap.
The prior deposition of amorphous silicon, which has been altered by hydrogen from the silane gas in an attempt to make it more closely resemble crystalline silicon and which has been doped in a manner like that of doping crystalline silicon, has characteristics which in all important respects are inferior to those of doped crystalline silicon. Thus, inadequate doping efficiencies and conductivity were achieved especially in the p-type material, and the photovoltaic qualities of these silicon alloy films left much to be desired.
Greatly improved amorphous silicon alloys having significantly reduced concentrations of localized states in the energy gaps thereof and high quality electronic properties have been prepared by glow discharge as fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980, and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, which are incorporated herein by reference, fluorine is introduced into the amorphous silicon semiconductor-to substantially reduce the density of localized states therein. Activated fluorine especially readily diffuses into and bonds to the amorphous silicon in the amorphous body to substantially decrease the density of localized defect states therein, because the small size of the fluorine atoms enables them to be readily introduced into the amorphous body. The fluorine bonds to the dangling bonds of the silicon and forms what is believed to be a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than is formed by hydrogen and other compensating or altering agents. Fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen because of its exceedingly small size, high reactivity, specificity in chemical bonding, and highest electronegativity. Hence, fluorine is qualitatively different from other halogens and so is considered a super-halogen.
As an example, compensation may be achieved with fluorine alone or in combination with hydrogen with the addition of these element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages so as to form a silicon-hydrogen-fluorine alloy. Such alloying amounts of fluorine and hydrogen may, for example, be in the range of 1 to 5 percent or greater. It is believed that the new alloy so formed has a low density of defect states in the energy gap than that achieved by the mere neutralization of dangling bonds and similar defect sates. Such larger amount of fluorine, in particular, is believed to participate substantially in a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to its other characteristics mentioned herein, is believed to be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. It is believed that fluorine also influences the bonding of hydrogen by acting in a beneficial way to decrease the density of defect states which hydrogen contributes while acting as a density of states reducing element. The ionic role that fluorine plays in such an alloy is believed to be an important factor in terms of the nearest neighbor relationships.
As disclosed in the aforementioned referenced U.S. Pat. No. 4,217,374, new and improved amorphous semiconductor alloys and devices can be made which are stable and composed of chemical configurations which are determined by basic bonding considerations. One of these considerations is that the material is as tetrahedrally bonded as possible to permit minimal distortion of the material without long range order. Fluorine, for example, when incorporated into these alloy materials, performs the function of promoting tetrahedral bonding configurations. Amorphous semiconductor materials having such tetrahedral structure exhibit low densities of dangling bonds making the materials suitable for photovoltaic applications.
Hydrogen, while smaller than fluorine, is by far less reactive. Hydrogen, as a result, in addition to promoting tetrahedral bonding, also promotes other various possible bonding configurations which can introduce defects into the material. For example, H.sub.2 Si bonds are possible. These bonds are weak bonds which can thermally be broken leaving behind dangling bonds. Also, hydrogen requires rather precise substrate temperature control during deposition to promote the preferred tetrahedral bonding. Therefore, hydrogen in small amounts, in conjunction with fluorine in small amounts should make the optimal amorphous semiconductor alloy. It is not hydrogen as a molecule or fluorine as a molecule, however, which is able to perform these functions. It is atomic hydrogen and atomic fluorine which does. From a chemical point of view in the plasma these species exist as free atoms or free radicals.
Amorphous semiconductor alloys made by the aforedescribed processes have demonstrated photoresponsive characteristics ideally suited for photovoltaic applications. These prior art processes however suffered from relatively slow deposition rates and low utilization of the reaction gas feed stock which are important considerations from the standpoint of making photovoltaic devices from these materials on a commercial basis. To alleviate the problem of slow deposition rate, a microwave plasma deposition processes was invented by Ovshinsky, et al. See U.S. Pat. No. 4,517,223, Method of Making Amorphous Semiconductor Alloys and Devices Using Microwave Energy, which issued on May 14, 1985, the disclosure of which is hereby incorporated by reference. The microwave plasma process herein provides substantially increased deposition rates and reaction gas feed stock utilization. Further, the microwave process resulted in the formation of reactive species not previously obtainable in sufficiently large concentrations with other processes. As a result, new amorphous semiconductor alloys could be produced having substantially different material properties than previously obtainable.
The principal advantage that microwave (e.g., 2.45 GHz) generated plasmas have over the more widely used radio frequency (e.g., 13.56 MHz) generated plasmas is that the thin film deposition rates are generally higher by factors of 10 to 100. This enhanced rate comes about because of the increased fraction of all the electrons in the plasma which have the necessary energy to be chemically relevant, i.e., energies greater than about 3 to 4 eV in which to excite molecules, and especially energies greater than 8 to 12 eV in which to dissociate and/or to ionize molecules. In other words, for the same power densities, microwave plasmas are more effective in generating the chemically active species (i.e., excited molecules, dissociated, very reactive radicals, ions, etc.), which are necessary to form thin films, than are radio frequency plasmas. One obstacle that has prevented thin film microwave plasma deposition from being more widely used is that the thin film material properties (e.g., electronic, density, etc.) tend to be inferior to those deposited by radio frequency plasma.
More recently, dual-frequency plasma deposition has been investigated as a method for simultaneously combining the advantageous features of both microwave and radio frequency plasma depositions. That is, while microwave plasma is very efficient for generating active. species in the gas phase (because of a relatively higher population of electrons in the energetic tail of the electron energy distribution function), resulting in deposition rates an order of magnitude higher than those observed at lower frequencies, the quality of the deposited material is lacking. On the other hand, radio frequency plasma is characterized by a negative d.c. bias at the cathode, which controls the flux and energy of ions impinging on the cathode surface, resulting in high quality thin film deposition, but at very low deposition rates. Dual-frequency plasma deposition using both microwave energy at 2.45 GHz and radio frequency energy at 13.56 MHz has been shown to produce better quality thin films than microwave energy alone at a higher deposition rate than radio frequency energy alone. See Klemberg-Sapieha et al, "DUAL MICROWAVE-R.F. PLASMA DEPOSITION OF FUNCTIONAL COATINGS", Thin Solid Films, vol. 193/194, December, 1990, pages 965-972.
Although this dual-frequency (ELF) plasma deposition method clearly has advantages over single frequency microwave (MW) or radio frequency (RF) plasma, it clearly only a compromise. While DF plasma has higher deposition rates than RF plasma, the rates are unquestionably lower those of MW plasma. Also, while the quality of DF plasma deposited films is greater than those deposited by MW plasma, it is undoubtedly lower than those deposited by RF plasma. It has been found that at the higher deposition rates of MW plasma deposition, it becomes increasingly important that the active neutrals (i.e. chemically activated, electrically neutral species) have higher kinetic energy so that when they arrive at the film surface, they will have the necessary surface mobility to create a high quality film.
In the discussion of microwave plasma deposition above attention should be focused on the electrons, for it is the electrons, which excite, dissociate and ionize the gaseous molecules. One possible mechanism why microwave energy leads to higher electronic energy in the plasma is that the electrons can resonantly couple to the energy. For example, in typical microwave plasmas, the electron density can range from 10.sup.10 to 10.sup.11 per cm.sup.3. At these densities, the resulting plasma oscillation frequency is on the same order as the applied radiation, namely in the low GHz range. Although a tremendous amount of the energy of the high frequency radiation can be pumped into the electrons by resonantly coupling, little if any of that energy can be transferred into kinetic energy of the ions and neutral molecules. This is because the ions are too massive to respond to the high frequency oscillations of the microwave and/or radio frequency energy, and also the energy gain of the electrons cannot be effectively transferred to the ions/neutrals due to the their large difference in mass.
There are two ways to resonantly couple energy into a plasma: (a) by high frequency radiation, which couples into the electrons; and (b) by low frequency radiation, which couples into the ions. When energy is pumped into the ions, their energy is very effectively transferred into the neutrals owing to their similarity in masses. Heating of the neutrals is desirable because it is the neutrals which overwhelmingly account for the deposition rate. It should be noted that because a relatively few ions must heat a large number of neutrals, efficiency requirements dictate the need to resonantly couple energy into the ions.
Large scale uniform deposition in rf plasma systems is not a serious issue because the wavelength at 13.56 MHz is more than 22 m, i.e., much greater than any practical deposition chamber. However, since the wavelength at the microwave frequency of 2.45 GHz is only 12.2 cm, the technical challenges in achieving uniform depositions over the desired scale of 30 cm are evident. Because of this, deposition results (e.g., film quality from a certain point in parameter space) obtained from small sized microwave research reactors (.about.5 cm substrates), cannot be automatically applied to the larger production systems because the hardware implementation will be fundamentally different. Scaling the process to larger sizes is not simply a matter of making larger electrodes (as it is in rf plasma systems) but involves new concepts. Although others have demonstrated high quality a-Si:H films with solar cell efficiencies approaching the best rf deposited material, similar results on the large production scale have yet to be demonstrated.
In the past, two types of microwave applicators were developed for different purposes. The first was a "single point" applicator, and the second was a "multi point" or "linear" applicator. The term "single point" refers to the fact that the microwaves emanate from only one region of space whose dimension is on the order of one wavelength. On the other hand, the "multi point" term refers to a system whereby the microwave energy emanates over a region of space extending over many (or multiple) wavelengths.
A) Microwave Feed-Through
The single point Applicator was developed for the purpose of coating xerographic drums. This device serves two basic purposes: (1) as a vacuum microwave feed-through or window; and (2) as the actual emanation or "launch" area where microwave energy is input into the plasma region. It consists of two alumina disks and one vitreous silica "quartz" disk which facilitates water cooling. Microwave energy enters from the air side and passes first through the quartz disk, and then through the two alumina disks before entering into the vacuum deposition chamber. The first alumina disk serves as the primary vacuum sealing component. The second alumina disk is in intimate contact with the first, and is directly exposed to the plasma region. Since this disk is removable, any build-up occurring during the deposition process can be easily removed. As the microwave energy passes through the window, into the plasma region, its energy is consumed in the plasma generation process, so that the microwave energy available for plasma production falls off exponentially in distance from the window. Thus, the corresponding drop in plasma intensity results in a fall off of the deposition rate with distance from the window. In the xerography application, deposition uniformity over the length of the drum required that two windows be used, one at each end of the drum.
This MW feed-through device was not designed as a high transmission device, i.e., one matching the transmission line (wave guide) impedance to the impedance of the deposition region. This was because of the difficulty in matching a fixed impedance device (wave guide) to the highly variable impedance of a plasma. It was however designed to withstand the high temperature effects associated with direct exposure to high power (about 5 kW or more) microwave plasmas. As microwave energy travels down the wave guide, the feed-through reflects some of the energy simply because of the change in impedance (i.e., refractive index) caused by the dielectric (quartz and alumina) plates. But the plasma will further modify the total input impedance so that the reflection of energy is not solely determined by the feed-through characteristics. Note that basic tuning can be accomplished with a 4-stub tuner located between the feed-through and the microwave generator. But even though it is possible to reduce the reflected power to very low levels, e.g., &lt;100 W as measured on the microwave generator side of the tuning device, this does not always assure that more energy is actually being delivered into the plasma region, for much of the tuning action of the tuner tends to be resistive (loss of power) in nature.
In addition to the successful use of this Applicator (feed-through) in xerographic drum production, it has also been used successfully in small research reactors and capable of producing high quality a-Si:H films for solar cells. But geometrical considerations make adapting this applicator to large area roll to roll solar cell production method unsuitable. In addition, more than one microwave generator would be required.
B) Microwave Linear Applicator
The multi point or linear applicator was developed for the coating of dielectric films over dimensions much larger than the microwave wavelength using a single power source. This device consists of a section of wave guide that contains a number of apertures on one of its faces. The efficiency of radiation at each point is determined by the size of the aperture from which the microwaves emanate. Uniformity of the radiation is therefore controlled by adjusting the aperture sizes. As microwaves enter into the Applicator, the energy density is greatest for the first aperture that is encountered. Because some energy has radiated away, the amount of energy traveling along the guide is diminished. The energy density at the next aperture is, therefore, less than at the first. To ensure that the total energy radiated from the second aperture is similar to the first, its size must be correspondingly larger. Uniformity along the length of the applicator is obtained by having progressively larger apertures.
This linear applicator is installed within a large vitreous silica (quartz) tube which serves as the vacuum seal/microwave feed-through. The inside of the tube is at atmospheric pressure while the outside is sealed to the vacuum deposition chamber, such that the applicator is actually at atmospheric pressure. Since the wave guide is directly connected to the applicator, and since the applicator is essentially a piece of wave guide, there is only a small impedance mismatch between them. This means that a tuning device is not needed between the applicator and the microwave generator.
Since the outside surface of the quartz tube is directly exposed to the plasma deposition region, the tube will become coated with whatever materials are being deposited within the chamber. When the depositing materials is a dielectrics such as SiO.sub.2 or Si.sub.3 N.sub.4, there are no detrimental consequences from the deposition thereof onto the quartz tube, due to the non microwave absorbing nature of these materials. However, for materials such as a-Si:H and a-Si:Ge:H alloys, even small deposited layers cause microwave absorptions which can lead to excessive heating of the quartz tube. This heating can eventually lead to crystallization of the deposited material which, in turn, dramatically increases the microwave absorption of the deposited layer. Failure of the quartz tube becomes imminent when the deposited material heats up to the melting point of quartz which can be catastrophic when dealing with pyrophoric silane gases.
Another consideration when using the quartz tube involves its serviceability. Since the large tube is a major vacuum component of the deposition system, replacement of the tube requires the system to be He leak checked. This time consuming process would be required before each production run.
In summary, the single point applicator (microwave feed-through) is a rugged and safe way to introduce high power microwaves into the low pressure deposition chamber. However, it is not particularly suited for large area depositions and, in any event, would require more than one microwave generator to accomplish large area deposition. On the other hand, the multi point (linear) applicator requires only one microwave generator for large area depositions, but would involve a dangerous use of a microwave window when producing a-Si:H and a-Si:Ge:H films.
Therefore, there is still a strong felt need in the art for a rugged and safe method of applying microwaves to the plasma deposition region in the vacuum chamber for large area deposition. There is also still a need in the art for a large area deposition method which combines the very high deposition rate of microwave plasma deposition with the high quality deposited films of RF plasma deposition.